In the field of lithographic printing, ink is continuously conveyed from an ink source through a series of rollers to a printing plate on a plate cylinder in a printing press. Image portions of the printing plate accept ink from one or more of the last of a series of inking rollers and transfer a portion of that ink to a blanket cylinder as a reverse image from which a portion of the ink is transferred to form a correct-reading image on paper or other materials. It is also important in conventional lithographic printing processes that a dampening solution containing water and proprietary additives be conveyed continuously to the printing plate whereby transferring in part to the non-image areas of the printing plate the water functions to keep those non-image areas free of ink. Finally, in conventional printing press systems, the ink is continuously made available in varying amounts determined by cross-press column input control adjustments to a plurality of ink metering devices, such as ink injectors. Open fountain inker systems, and other systems, may also be used as ink metering devices.
Lithographic printing plate surfaces in the absence of imaging materials have minute interstices and a hydrophilic or water-loving property to enhance retention of water, that is the dampening solution, rather than ink on the surface of the plate. Imaging the plate fills these interstices and creates oleophilic or ink-loving areas according to the image that is to be printed. Consequently, when both ink and dampening solution are presented to an imaged plate in appropriate amounts, only the ink tending to reside in non-image areas becomes disbonded from the plate. In general, this action accounts for the continuous ink and dampening solution differentiation on the printing plate surface, which is integral to the lithographic printing process.
During a make-ready or set up process, a printing press is prepared for a new print job. In this regard, a new print job refers to printing different images on the web as compared to an existing print job. This can be accomplished, for example, by changing the printing plate(s) on a printing unit, or by bringing a different set of printing units into contact with the web. Both require make-ready, although in the latter case, sometimes referred to as auto-transfer, the make-ready for the new print job could be performed at any time prior to the job change. In any event, during this make-ready (or set up) process, the press is adjusted and stabilized before it is ready to produce an accurate and acceptable image on the printed material. For example, adjustments are made to the press color and/or registration during start up. Thereafter, the press is run for a period of time needed for the effect of the adjustments to propagate through to the printed substrate, often referred to as the run-in time. During the run in time, the images on the printed substrate are not usable, and are often referred to as “waste.” The press may need to be stopped and started a number of times as make-ready adjustments are iteratively made until acceptable print quality is achieved.
One aspect of the make-ready process is ink stabilization. In this regard, during the run-in period noted above, it is generally necessary to operate the press, applying ink and water to the printing plate and transfer the image from the plate to the blanket in order to stabilize the ink transfer process so that the desired ink thickness, typically measured by optical density, is achieved.
During this period, ink will build up on the blanket and must be removed. This traditionally has been accomplished by running a sufficient amount of waste material through the press so as to stabilize the transfer of the ink image by the press. This cycle is repeated each time an adjustment to color or registration is made on the press.
Conventionally, inker make-ready on offset presses has been accomplished by running substrate continuously at the same surface velocity as the inker.
Open loop ink key forcing functions have been applied to improve the natural response of the inker, but substrate is still run through the press during this period resulting in undesirable waste.
U.S. Pat. No. 5,235,913 purports to describe a device and method for stabilizing an offset lithographic printing press. A litho start-off device comprises an ink removal cylinder which can be selectively engaged and disengaged with a blanket disposed on the surface of the blanket cylinder. When engaged with the blanket cylinder, the ink removal cylinder removes ink from the blanket of the blanket cylinder. A scraper assembly is also provided to remove the ink from the ink removal cylinder as it rotates.